Single crystals and methods for fabricating same

ABSTRACT

Various single crystals are disclosed including sapphire. The single crystals have desirable geometric properties, including a width not less than about 15 cm and the thickness is not less than about 0.5 cm. The single crystal may also have other features, such as a maximum thickness variation, and as-formed crystals may have a generally symmetrical neck portion, particularly related to the transition from the neck to the main body of the crystal. Methods and for forming such crystals and an apparatus for carrying out the methods are disclosed as well.

CROSS-REFERENCE TO RELATED APPLICATION(S) BACKGROUND

1. Field of the Invention

The present invention is generally drawn to single crystal components,and particularly to single crystal sheets, methods for forming suchsheets, and processing equipment used in connection with the formationof single crystal sheets.

2. Description of the Related Art

Single crystals such as sapphire have been a material of choice fordemanding, high performance optical applications, including variousmilitary and commercial applications. Single crystal sapphire possessesgood optical performance within the 200 to 5000 nanometer range, andadditionally possesses desirable mechanical characteristics, such asextreme hardness, strength, erosion resistance, and chemical stabilityin harsh environments.

While certain demanding high performance applications have takenadvantage of single crystal sapphire, its implementation has not beenwidespread partly due to cost and size limitations due to formingtechnologies. In this regard, single crystal sapphire in the form ofsheets is one geometric configuration that holds much industrialpromise. However, scaling size while controlling processing costs hasbeen a challenge in the industry. For example, processing equipment hasnot been adequately developed for the repeatable production oflarge-sized sheets, and additionally, processing techniques have notbeen developed for reliable manufacture.

A publication entitled “Large Diameter Sapphire Window from SingleCrystal Sheets” from the Proceedings of the Fifth DOD ElectromagneticWindow Symposium, Volume I (October 1993) provides a description ofsapphire sheet processing (co-authored by the present inventor).However, the technology described in the paper is confined, particularlylimited to moderate sheet sizes.

In light of the foregoing, the industry continues to demand large-sizedsingle crystal sheets that can be produced in a cost-effective manner,such that improved size and reduced cost enable the implementation ofsheets in various applications that, to date, have not been exploited.In addition, there is a particular demand for large-sized sapphiresheets.

SUMMARY

According to a first aspect of the present invention, a sapphire singlecrystal is provided. The sapphire single crystal is in the form of asheet having a length>width>>thickness, the width not being less than 15centimeters and the thickness being not less than about 0.5 centimeters.

According to another aspect, a sapphire single crystal is provided, inthe form of a sheet having a length>width>thickness, the width being notless than 15 centimeters and a variation in thickness of not greaterthan 0.2 centimeters.

According to yet another aspect, a sapphire single crystal is provided,comprised of an as-grown single crystal sheet having a main body and aneck. The main body has first and second opposite lateral sides that aregenerally parallel to each other, a transition of the neck to the mainbody being defined by respective ends or transition points of the firstand second opposite lateral sides. According to a particular feature,the single crystal sheet has a Δ_(T) that is not greater than 4.0centimeters. Δ_(T) is the distance by which the first and secondtransition points are spaced apart from each other, as projected along alength segment of the single crystal sheet.

According to yet another aspect, a method of forming a single crystal isprovided in which a melt is provided in a crucible having a die. Thethermal gradient along the die is dynamically adjusted, and a singlecrystal is drawn from the die.

According to another aspect, a method of forming a single crystal isprovided, including providing a melt, drawing a single crystal from thedie, and pulling the single crystal upward from the die and into anafterheater. The afterheater has a lower compartment and an uppercompartment separated from each other by an isolation structure.

According to yet another aspect, a method of forming a single crystal isprovided, including providing a melt in a crucible of a melt fixture.The melt fixture has a die open to the crucible and a plurality ofthermal shields overlying the crucible and the die, the thermal shieldshaving a configuration to provide a static temperature gradient alongthe die, such that temperature is at a maximum at about the midpoint ofthe die. The single crystal is drawn from the die.

According to another aspect, melt fixtures are also provided. In oneaspect, the melt fixture has a shield assembly providing a desirablestatic temperature gradient. In another aspect, the melt fixtureincludes an adjustable gradient trim device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate front and side schematic views of a crystalgrowth apparatus according to an embodiment of the present invention.

FIG. 3 illustrates an end perspective view of a melt fixture accordingto an embodiment of the present invention, which forms a component ofthe crystal growth apparatus shown in FIGS. 1 and 2.

FIG. 4 illustrates dimensions of the crucible of the melt fixture shownin FIG. 3.

FIG. 5 illustrates an elevated perspective view of the melt fixtureshown in FIG. 3.

FIG. 6 shows two as-grown sapphire single crystals.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

According to various embodiments of the present invention, new sapphiresingle crystals, a crystal growth apparatus, particularly, an EFG growthapparatus, and methods for growing single crystals are provided.Description of these various embodiments begins with a discussion of theEFG growth apparatus 10 illustrated in FIGS. 1 and 2. As used herein,the term EFG refers to the Edge-Defined-Film-Fed Growth technique, atechnique that is generally understood in the industry of single crystalfabrication, and particularly include EFG as applied to sapphire singlecrystal growth.

Turning to FIGS. 1 and 2, EFG growth apparatus 10 includes several maincomponents, including a pedestal 12 supporting melt fixture 14, which isopen to and communicates with afterheater 16. Pedestal 12 is generallyprovided to mechanically support the apparatus while thermally isolatingthe melt fixture 14 from the work surface on which the EFG apparatus isprovided, to attenuate heat transmission from the melt fixture 14 to thework surface. In this context, the pedestal 12 is generally formed of arefractory material capable of withstanding elevated temperatures on theorder of 2,000° C. While various refractory metals and ceramics may beutilized, graphite is particularly suited for the pedestal 12. Ventholes 16 are provided in pedestal 12 to further improve thermalisolation.

Turning to melt fixture 14, crucible 20 is provided for containing themelt that is utilized as the raw material for forming the singlecrystal. In the context of sapphire single crystals, the raw material isa melt from alumina raw material. The crucible 20 is typically formed ofa refractory metal that is adapted to be heated through exposure to thefield generated by an inductive heating element 17. The crucible isdesirably formed of molybdenum (Mo) although other materials may beutilized such as tungsten, tantalum, iridium, platinum, nickel, and inthe case of growth of silicon single crystals, graphite. More generallyspeaking, the materials are desired to have higher a melting point thanthe crystal being drawn, should be wet by the melt, and not reactchemically with the melt. The inductive heating element 17 illustratedis an RF coil, having multiple turns forming a helix. Within thecrucible 20, a die 18 is provided, which extends into the depth of thecrucible, the die 18 having a center channel that is open through acrucible lid 21 (see FIG. 3) and generally exposed to afterheater 16(described in more detail below). The die 18 is alternatively referredto as a “shaper” in the art.

Further, the melt fixture 14 includes a shield assembly 26 that isformed of a plurality of horizontal and vertical shields discussed inmore detail below. The melt fixture 14 is generally mechanicallysupported by a support plate 22 overlying pedestal 12. Thermalinsulation is provided by bottom insulation 24 as well as insulationlayers 32 generally surrounding the lateral sides and top of the meltfixture 14. The bottom insulation 24 and the insulation layers 32 may beformed of graphite felt, for example, although other insulationmaterials may be utilized such as low conductivity rigid graphite board(such as Fiberform from FMI Inc.); other materials are, whenthermodynamically compatible, alumina felt and insulating materials;zirconia felt and insulation; aluminum nitride, and fused silica(quartz). The shield assembly 26 includes horizontal shields 28 andvertical shields 30, which may also be seen in FIGS. 3 and 5.

The next major structural component of the EFG growth apparatus 10 isthe afterheater 16 that includes a lower compartment 40 and an uppercompartment 42. The upper and lower compartments are separated from eachother by an isolation structure. In the particular embodiment shown inFIGS. 1 and 2, the isolation structure illustrated is formed by lowerisolation doors 44. For illustration, one of the doors is provided inthe closed position and the other of the doors in the open position. Asecond isolation structure is also provided to separate the afterheater16 from the external environment. In the embodiment shown in FIGS. 1 and2, the upper isolation structure is formed by upper isolation doors 45.

While a more detailed discussion is provided below regarding the growthprocess and operation of the EFG growth apparatus, the process generallycalls for lowering a seed crystal 46 through the afterheater 16 to makecontact with the liquid that is present at the top of the die 18,exposed through the crucible lid and to the afterheater 16. In theembodiment illustrated, the afterheater is passive, that is, does notcontain active heating elements. However, the after heater may beactive, incorporating temperature control elements such as heatingelements. After initial growth, the seed crystal is raised and thegrowing single crystal 48 spreads to form a neck portion, having agrowing width but which is less than the length of the die. The neckportion spreads to full width, initiating the growth of the full widthportion or main body of the single crystal. The single crystal is thenraised through the afterheater, first through lower compartment 40 andthen into upper compartment 42. As the single crystal 48 translates intothe upper compartment 42, the isolation doors 44 automatically closebehind, thereby isolating the upper compartment 42 and the singlecrystal 48 from the lower compartment 40 and melt fixture 14.

The isolation structure in the form of the lower insulation doors 44provides several functions. For example, in the case of catastrophicfailure of crystal 48 during cooling, the resulting debris is preventedfrom impacting the relatively sensitive melt fixture 14. In addition,the isolation doors 44 may provide thermal insulation, to provide acontrolled cooling environment in the upper compartment 42, therebycontrolling cooling rate in the upper compartment 42.

In further reference to FIGS. 1 and 2 a gradient trim system 50 isprovided. Gradient trim system 50 functions to dynamically adjust athermal gradient along the length of the die 18. The gradient trimsystem 50 includes upper thermal shields 52 provided at opposite ends ofthe melt fixture, the thermal shields being adapted to be positioned atvarious heights by manipulating a linkage mechanism to adjust thethermal gradient along the length of the die. The thermal shields 52 maybe embodied to function as active heating elements through inductionheating by inductive heating element 17, or maybe embodied to reflectambient thermal energy. In the latter case, graphite sheets areparticularly useful such as Grafoil™ (such as available from FiberMaterials Inc. (FMI Inc.) of Biddeford, Me.).

Turning to FIGS. 3-5, various features of melt fixture 14 areillustrated. As shown, the melt fixture includes crucible 20 supportedby support plate 22. Further, the crucible 20 is closed off by cruciblelid 21, over which the shield assembly 26 is provided. Shield assembly26 includes the horizontal shields 28 as illustrated, which arepositioned by shield pins 29. Horizontal shields are shown generally asplates having a planar geometry, although other contours may beutilized. The horizontal shields are configured to shape the static(baseline) thermal profile, which can be further manipulated asdiscussed hereinbelow. The shields may be formed of a material toreflect thermal energy, or may actively be heated in the presence of theinductive field.

According to a particular feature, the horizontal shields 28 are dividedinto first and second shield sets respectively positioned along firstand second lateral sides of the die 18. Each of the shield sets aregenerally symmetrical about a vertical central axis. In the embodimentshown in FIGS. 3 and 5, the vertical central axis extends along thecentral bore of feed tubes 33, through which the raw material is fedinto the crucible to form the melt. Of particular significance, thehorizontal shields 28 form a stepped configuration forming opposingslopes that are oriented toward the central axis. As illustrated,adjacent overlying shield pairs are progressively shorter, to define thesloping and stepped configuration.

According to another feature, the crucible has an elongated structure,that is, has a structure in which the horizontal cross section is notcircular. In reference to FIG. 4, the crucible has a length l, a widthw, and a depth d, wherein an aspect ratio defined as l:w is not lessthan 2:1. As shown in FIG. 4, the length and width of the crucible 20are mutually perpendicular, and represent the internal dimensions of thecrucible. According to certain embodiments, the aspect ratio is not lessthan 3:1, such as not less than 4:1. While the cross-sectional shape ofthe crucible 20 is generally oval, other embodiments may be rectangularor polygonal, while still maintaining the foregoing aspect ratiofeatures. The inductive heating element 17 shown in FIGS. 1 and 2 alsohas an aspect ratio similar to that of the crucible, namely beinggreater than 2:1. A comparison of the length of the coils shown in FIG.1 to the width of the coils shown in FIG. 2 illustrates this feature.

Now focusing on operation of the EFG growth apparatus 10, typicallycrystal growth begins with formation of a melt in the crucible. Here,the crucible is filled with a feed material, Al₂O₃ in the case ofsapphire. The feed material is generally provided by introductionthrough the feed tubes 33. The melt is initiated and maintained byinductive heating at a temperature of about 1950° C. to about 2200° C.,by energizing inductive heating element 17 having a plurality ofinductive heating coils. Heating by induction is effected by heating ofthe crucible 20, transmitting thermal energy into the material containedtherein. The melt wets the die 18, forming a layer of liquid at thesurface of the die.

After formation of a stable melt in the crucible, the seed crystal 46 islowered through the afterheater 16, to contact the liquid at the dieopening. After contact of the seed crystal with the melt at the dieopening, the liquid film of the melt extending from the die to the seedis observed and temperature and temperature gradient (discussed below)are adjusted to reach a film height, such as on the order of 0.3 to 2.0millimeters. At this point, the seed crystal is slowly raised such thatupon raising the crystal into the lower compartment of the afterheater40 the lower temperature causes crystallization of the liquid melt,forming a single crystal. The seed crystal is generally raised within arange of about 3 to 30 centimeters per hour, such as within a range of 3to 15 centimeters per hour or 3 to 10 centimeters per hour.

At this point in the crystal growing process, a neck is grown,representing a sub-maximum width of the single crystal. Turning brieflyto the full-length single crystal 100 shown in FIG. 6, the singlecrystal 100 includes a main body 102 and a neck 104, wherein thetransition from the neck to the main body is labeled T. The initialportion of the neck extending from the distal end 106 is desired to havea minimum geometry, such as on the order of a few centimeters long, andhaving a thickness corresponding to at least one half the width of thedie. Once assuring that the initial growth of the neck is desirable, thebalance of the neck is grown by lowering the pull speed to be on theorder of 0.1 cm/hr to about 20 cm/hr, often times within a range ofabout 0.1 cm/hr to about 10 cm/hr, more particularly, 0.5 cm/hr to 5cm/hr. Additionally, the temperature may be lowered to be on the orderof 10° C. to 100° C. lower, such as 110° C. to 50° C. lower than theinitial starting temperature of the process.

Upon continued pulling of the seed crystal 46, the neck widens tomaximum width, which is the length of the die 18. Of significance, it isdesired that the neck spreads uniformly and symmetrically to oppositeends of the die during the pulling process, such that the heightdifference between the initiation of the main body portion defined bythe transition of opposite lateral sides of the main body, are withinabout 4 centimeters of each other, as projected along the verticalheight of the crystal.

Turing back to FIG. 6, an illustration of two difference crystals isprovided, demonstrating a difference in spread uniformity. The firstcrystal 80 represents a partial neck portion of a rejected (out-of-spec)single crystal, while crystal 100 represents an as-formed full-lengthcrystal that was acceptable for further processing into usefulcomponents. Crystal 100 includes a main body 102 and a neck 104, whereinthe transition from the neck 104 to the main body during growth happensalong transition zone T as labeled. Generally, the neck 104 has anincreasing thickness form the distal end 106 to the transition zone T.As shown, the main body 102 includes first and second opposite lateralsides 108, 110 that are generally parallel to each other, wherein theend point of each of the sides 108 and 110 is defined by respectivelateral transitions from the neck 104 to the main body 100, representingfull width. The fist lateral side 108 includes an end point defined bytransition point 112, and similarly, lateral side 110 includes an endpoint defined by transition point 114. The transition points 112, 114are projected perpendicularly along a length segment (or long axis) ofthe single crystal sheet and the distance by which the respectivetransition points 112 and 114 are spaced apart along that length arerepresented by Δ_(T), representing a difference in heights betweentransition points of the opposite lateral sides of 108 and 110 of themain body 102. Desirably, Δ_(T) is not greater than about 4.0centimeters, such as not greater than about 3.0 centimeters, inparticular, not greater than about 2.0, 1.5, 1.0, 0.8, or even 0.7centimeters. Ideally, Δ_(T) is zero although practically a zero delta isdifficult to reproduce.

If the Δ_(T) is greater than a predetermined spec, such as 4.0centimeters, the crystal is pulled free from the melt, discarded, and agrowth operation is reinitiated. An out of spec crystal is illustratedin FIG. 6 as crystal 80.

An excessively high Δ_(T) generally corresponds to undesirable thicknessvariations across the width of the crystal, causing internal stressesand attendant low yield rates, as well as processing issues infabrication of optical components from the crystal. High Δ_(T) isrelated to high thermal gradients along the length of the die.Accordingly, pursuant to a particular feature, the thermal gradientalong the length of the die is adjusted to provide for growth of asingle crystal having a Δ_(T) that is within spec.

Turning back to FIGS. 1 and 2, the thermal gradient may be adjusted bymanipulating the gradient trim system 50 having first and second heatshields provided at opposite ends of the die. In the particularembodiment shown in FIGS. 1 and 2, raising a shield at a particular endraises the temperature at that end, while lowering the shield lowers thetemperature at that end of the die. Temperature readings (e.g., from apyrometer or thermocouples) along the length of the die guide theadjustment of gradient trim system 50. Typically, the thermal gradientis reduced to a value of not greater than about 0.6° C./cm along alength of the die during the drawing operation. Other embodiments havean even further reduced thermal gradient such as on the order of 0.4, oreven 0.3° C./cm. Alternatively, the thermal gradient is reduced to avalue of not greater than about 20° C., such as not greater than 15° C.between the first and second opposite ends of the die during drawing.

The overall temperature profile along the length of the die is generallysuch that the center of the die has the highest temperature, withtemperature falling off to the edges of the die. Ideally, the curve issymmetrical, where temperature from the center to each end of the diefalls off uniformly, creating generally similar temperature gradientsfrom the center of the die to each end of the die. Noteworthy, the shapeof the shield assembly (discussed above), is chosen to provide thedesired static shape of the temperature profile. As such, the shields,acting as heating elements are typically symmetrical about an axisbisecting the die, and have a height that is at its maximum at thecenter of the die, gradually decreasing to a minimum at opposite ends ofthe die.

Typically, the adjustment is carried out prior to growth of a singlecrystal, which includes adjustment between growth of individual singlecrystals, such as between the growth of the first single crystal 80 andthe growth of the second single crystal 100. In either case, dynamicadjustment of the thermal gradient is typically carried out after theformation of the melt in the crucible. Still further, the thermalgradient may be adjusted during the growth of the single crystal, thatis, during the pulling of the seed crystal so as to grow and draw thesingle crystal.

While adjustment of the thermal gradient has been described herein inconnection with use of the gradient trim system 50 that includes thermalshields, other gradient trim systems may be utilized. For example,thermal shields may be replaced with heat sinks, which act to draw heataway from the die. In the manner known in the art, heat sinks may takeon the form of a heat exchanger, such as those that have a fluid flowingtherethrough for carrying thermal energy away from the heat sink. Theamount of thermal energy drawn away from either end of the die may beadjusted by manipulating the temperature of the fluid flowing throughthe heat exchanger, such as through use of a thermostat provided in-linewithin the fluid flow system, and/or adjusting flow rates.Alternatively, the position of the heat sink may be adjusted to modifythe amount of thermal energy drawn from the respective end of the die.

Upon the creation of a full-length single crystal having a ΔT that iswithin spec, the single crystal is broken free from the melt by pulling,and temperature is stabilized by maintaining the single crystal withinthe lower compartment 40 of the afterheater 16. Thereafter, the singlecrystal is pulled to enter upper compartment 42, during which acontrolled cooling of the crystal is effected. Typically, cooling iscarried out at a rate not greater than about 300° C./hr, such as notgreater than about 200, 150, or even 100° C./hr. According to anembodiment, the cooling rate is not less than about 50° C./hr., such aswithin a range of about 50 to 100° C./hr. The relatively slow coolingrates are generally dictated by several parameters, including the massof the crystal. Here, in the case of relatively large single crystals,it is not uncommon that the single crystal to have a mass greater thanabout 4 kg, such as greater than about 5 or 6 kg, such as on the orderof 7 kg.

Following the drawing and cool down of the single crystal, machiningoperations are typically effected. It is generally desired that thesingle crystal be near-net shape, but oftentimes machining is effectedto form the single crystal into the desired geometric configurations forcommercial use. Accordingly, grinding, lapping, polishing and the like,or bulk material removal/shaping such as wire sawing or cleaving and thelike may be utilized to manipulate the single crystal into a desiredcomponent or components, such as optical windows for bar codes scanners,optical windows for infrared and laser guidance, sensing and targetingsystems in military operations, optical windows for infrared and visiblewavelength vision systems. The optical window in such implementationsmay function to act as a window that is scratch and erosion resistantwhile being transparent in the infrared and visible wavelengthspectrums. Other applications include transparent armor, such as bulletresistant windshields made of composites that include large sheets ofsapphire.

Turning to the single crystal itself, the single crystal is in the formof alumina single crystal (sapphire). Typically, the single crystal isrelatively wide, such as having a width not less than about 15 cm, suchas not less than about 17, 20, 22, 25, or even 28 cm. The widthcorresponds to the length of the die during the drawing operation, thedie determining the desired maximum width of the crystal. Further,according to a particular feature, the average thickness is not lessthan about 0.5 cm, such as not less than about 0.6, 0.7, 0.8, or even0.9 cm.

Further, the single crystal typically has a relatively confinedvariation in thickness, having a variation not greater than about 0.2cm. Here, variation in thickness corresponds to the maximum thicknessvariation along a segment spanning the width of the main body of thesingle crystal sheet. Ideally, the maximum thickness variationcorresponds to substantially the majority of all width segments alongthe main body, generally denoting a maximum thickness variation alongthe majority of main body of the single crystal.

EXAMPLES

Example 1, a crystal having dimensions 305±3×475±10×9.3±0.8 (W×L×T inmm). The following process flow was used to form Example 1.

-   a. Set up furnace with growth components: crucible, die, shields and    insulation package (hotzone).-   b. Purge chamber for 2 hours at 40 scfm Argon.-   c. Turn on power 150 kW supply.-   d. Ramp power @ 0.625% per minute to a temperature set point of    1950° C.-   e. Manually adjust temperature until melting (T_(m)) is observed.-   f. Manually adjust temperature from T_(m) to T_(m)+60° C.-   g. Start feeder and add 4100 g feed material into crucible.-   h. Allow melt to stabilize for 1 hour.-   i. Lower seed and contact die at mid point.-   j. Adjust temperature so that approximately 1 mm of liquid film    separates seed crystal and die (T_(n)).-   k. Start upward translation of puller at 75 mm/hr.-   l. Grow neck of crystal for 25 mm, inspect for uniform cross section    and sufficient width, such as approximately 1/2 the width of the    die. If neck is not uniform, break crystal free and adjust    temperature gradient and reinitiate growth process for a new    crystal.-   m. Adjust temperature to T_(n)−40° C. and lower pull speed to 25    mm/hr.-   n. Allow crystal to spread to edges of die. If crystal does not    spread uniformly to edges of die, break crystal free and adjust    temperature gradient and reinitiate growth process for a new crystal-   o. Start feeder once 50 mm length of growth has been reached and add    feed material at a rate of 2.2 g/min—for a total of 2250 g over the    course of the growth run.-   p. Adjust the temperature and/or temperature gradient to maintain    uniform liquid film height of 0.3±0.1 mm at die interface while    growing body of crystal at 25 mm/hr.-   q. When full width crystal has reached a length of 485 mm pull the    crystal free of die by increasing pull rate to 7500 mm/hr for a    length of 8 mm.-   r. When bottom of crystal reaches 8 mm above die lower the pull rate    to 150 mm/hr until crystal bottom is 150 mm above the die.-   s. Increase pull speed to 375 mm/hr until crystal emerges from hot    zone at the top of the furnace.

Other Examples

For different size crystals, the amount of raw material fed into themelt fixture over the growth period changes to accommodate the differentweight of the crystal. For example, the total weight for Example 1 wasabout 6350 g. For a 230×610×9.3 the total weight will be 6150 g. So inthis second example, the initial charge is 4100 g and the amount chargedin would be 2050 at 1.5 g/min (2050 grams/˜24 hour growth (610 mm/25mm/hr)). Generally, it is desirable to charge the incoming raw materialgenerally uniformly through the growing process, over the whole lengthof the crystal.

Through use of various features of the embodiments of the presentinvention, such as utilization of a high aspect ratio crucible, highaspect ratio heating element, use of a gradient trim system, andintroduction of a compartmentalized afterheater, sapphire single crystalsheets having the foregoing desirable geometric and mass features suchas minimum width, thickness, and thickness variation features may besuccessfully formed. More particularly, use of a high aspect ratiocrucible may improve process uniformity and repeatability, which use ofa thermal gradient system for dynamically controlling the thermalgradient along the length of the die can be used to minimize the thermalgradient along the die, maximum temperature variations along the die,and accordingly provide for a symmetrical spread along the neck of thesingle crystal, contributing to thickness uniformity and the ability togrow relatively large mass and relatively thick crystals. While theprior art has reported success in the formation of moderately sizedsingle crystals having limited width and/or thickness, embodiments ofthe present invention provide for improved process control and equipmentenabling next generation, large sized single crystals, and inparticular, single crystal sheets.

The above-disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments, which fall withinthe scope of the present invention. For example, while certainembodiments focus on growth of large-sized sapphire, other singlecrystals may be fabricated utilizing the process techniques describedherein. Thus, to the maximum extent allowed by law, the scope of thepresent invention is to be determined by the broadest permissibleinterpretation of the following claims and their equivalents, and shallnot be restricted or limited by the foregoing detailed description.

1. A sapphire single crystal, comprising: a single crystal sheet havinga length, width and thickness, wherein length>width>thickness, the widthis not less than about 15 cm, and the thickness is not less than about0.5 cm.
 2. The crystal of claim 1, wherein the width is not less thanabout 20 cm.
 3. The crystal of claim 1, wherein the width is not lessthan about 25 cm.
 4. The crystal of claim 1, wherein the thickness isnot less than about 0.6 cm.
 5. The crystal of claim 1, wherein thethickness is not less than about 0.7 cm (0.8, 0.9 in spec).
 6. Asapphire single crystal, comprising: a single crystal sheet having alength, wherein length>width>thickness, the width is not less than about15 cm, and a variation in thickness of not greater than 0.2 cm.
 7. Thecrystal of claim 6, wherein the variation in thickness is not greaterthan 0.15 cm.
 8. The crystal of claim 6, wherein the variation inthickness is not greater than 0.10 cm.
 9. The crystal of claim 6,wherein the variation in thickness is not greater than 0.07 cm.
 10. Thecrystal of claim 6, wherein the variation in thickness is the differencebetween maximum and minimum thickness values along a segment spanningthe width of the sheet.
 11. A sapphire single crystal, comprising: asingle crystal sheet having a length, width and thickness, whereinlength>width>thickness, the width is not less than about 15 cm, thethickness is not less than about 0.5 cm, and a variation in thickness ofnot greater than 0.2 cm.
 12. A sapphire single crystal, comprising: anas-grown single crystal sheet having a length, width and thickness,wherein length>width>thickness, the width is not less than about 15 cm,the single crystal sheet further having a neck and a main body thatdefines first and second opposite lateral sides that are generallyparallel to each other, a transition of the neck to the main bodyportion is defined by respective first and second transition points ofthe first and second opposite lateral sides, and the single crystal hasa Δ_(T) not greater than 4.0 cm, wherein Δ_(T) is the distance by whichthe respective first and second transition points are spaced apart asprojected along a length segment of the single crystal sheet.
 13. Thecrystal of claim 6, wherein Δ_(T) is not greater than about 3.0 cm. 14.The crystal of claim 6, wherein Δ_(T) is not greater than about 2.0 cm.15. The crystal of claim 6, wherein the neck increases in thickness fromits distal end to the main body.
 16. A method of forming a singlecrystal, comprising: providing a melt in a crucible having a die;dynamically adjusting a thermal gradient along the die; and drawing asingle crystal from the die.
 17. The method of claim 16, wherein thestep of dynamically adjusting is carried out to reduce the thermalgradient along the die.
 18. The method of claim 17, wherein the thermalgradient is reduced to a value of not greater than about 0.6° C./cmalong a length of the die during drawing.
 19. The method of claim 18,wherein the step of dynamically adjusting is carried out prior todrawing, such that during drawing, the thermal gradient is reduced tosaid value.
 20. The method of claim 16, wherein the step of dynamicallyadjusting is carried out during the drawing step.
 21. The method ofclaim 16, wherein drawing a single crystal includes drawing a firstsingle crystal and drawing a second single crystal, wherein the thermalgradient is adjusted after drawing the first single crystal and beforedrawing the second single crystal.
 22. The method of claim 16, whereinthe single crystal is sapphire.
 23. The method of claim 16, wherein thedie is linear and the single crystal is in the form of a generallyplanar sheet, the die having first and second opposite ends defining alength, the length of the die corresponding to the width of thegenerally planar sheet.
 24. The method of claim 23, wherein the lengthof the die is not less than about 22 cm.
 25. The method of claim 23,wherein the length of the die is not less than about 25 cm.
 26. Themethod of claim 23, wherein the thermal gradient is adjusted bymanipulating a movable thermal shield positioned along at least one ofthe first and second opposite ends of the die.
 27. The method of claim26, wherein a movable thermal shield is positioned at both the first andsecond opposite ends of the die.
 28. The method of claim 26, wherein themovable shield functions to radiate heat toward the die.
 29. The methodof claim 23, wherein the thermal gradient is reduced to a value of notgreater than about 20° C. between first and second opposite ends of thedie during drawing.
 30. The method of claim 29, wherein the thermalgradient is reduced to a value not greater than about 15° C. betweenfirst and second opposite ends of the die during drawing.
 31. The methodof claim 16, wherein the thermal gradient is adjusted by manipulating aheat sink, to draw heat away from the die.
 32. The method of claim 31,wherein the heat sink is a heat exchanger.
 33. The method of claim 32,wherein the heat exchanger has a fluid flowing therethrough.
 34. Themethod of claim 33, wherein the heat sink is manipulated by modifying atleast one of flow of the fluid and re-positioning the heat sink.
 35. Themethod of claim 16, wherein the thermal gradient is adjusted bymanipulating a movable thermal shield.
 36. The method of claim 16,wherein the crucible is comprised of a refractory metal.
 37. The methodof claim 36, wherein the crucible comprises Mo.
 38. The method of claim16, wherein the melt is provided by heating using induction heating. 39.The method of claim 38, wherein induction heating is carried out byenergizing an inductive coil surrounding the crucible.
 40. The method ofclaim 16, wherein the coil forms a helix having multiple turns, the coilhaving a non-circular cross section having an aspect ratio of at least2:1.
 41. The method of claim 16, wherein the crucible has a non-circularhorizontal cross section.
 42. The method of claim 41, wherein thenon-circular cross section is oval or rectangular.
 43. The method ofclaim 41, wherein crucible has an aspect ratio not less than 2:1. 44.The method of claim 16, further comprising machining the single crystalinto components.
 45. A method of forming a single crystal, comprising:providing a melt; drawing a single crystal from the die; and pulling thesingle crystal upward from the die and into an afterheater, theafterheater having a lower compartment and an upper compartmentseparated from each other by an isolation structure.
 46. The method ofclaim 45, further comprising cooling the single crystal in the uppercompartment of the afterheater, the single crystal cooling at a rate notgreater than about 300° C./hr.
 47. The method of claim 46, wherein therate is not greater than 200° C./hr.
 48. The method of claim 46, whereinthe rate is not greater than 150° C./hr.
 49. The method of claim 46,wherein the rate is not greater than 100° C./hr (50-100 in spec). 50.The method of claim 45, wherein the single crystal is a generally planarsheet having a mass greater than about 4 kg.
 51. The method of claim 50,wherein the mass is greater than about 6 kg.
 52. The method of claim 45,wherein isolation structure comprises isolation doors that close behindthe crystal as the crystal passes into the upper compartment.
 53. Themethod of claim 45, further comprising machining the single crystal intocomponents.
 54. A method of forming a single crystal, comprising:providing a melt in a crucible of a melt fixture, the melt fixturehaving a die open to the crucible and a plurality of thermal shieldsoverlying the crucible and the die, the thermal shields havingconfiguration to provide a static temperature gradient along the die,such that temperature is at a maximum at about the midpoint of the die;and drawing a single crystal from the die.
 55. The method of claim 54,wherein thermal shields have a stepped configuration.
 56. The method ofclaim 54, wherein thermal shields include a first shield set positionedalong a first lateral side of the die, and a second shield setpositioned along an opposite, second lateral side the die.
 57. Themethod of claim 56, wherein each of the first and second shield sets aregenerally symmetrical about a vertical central axis corresponding tomidpoint of the die.
 58. A method of forming a single crystal,comprising: providing a melt in a crucible of a melt fixture, the meltfixture comprising a linear die open to and extending along a length ofthe crucible, and a plurality of thermal shields overlying the crucibleand the die, the thermal shields having a configuration to provide astatic temperature gradient along the die, such that temperature is at amaximum at about the midpoint of the die, and the crucible having anaspect ratio of at least 2:1, the aspect ratio being defined as theratio of the length of the crucible to the width of the crucible;dynamically adjusting a thermal gradient along the die; and drawing asingle crystal from the die.
 59. A melt fixture, comprising a crucible;a die open to and extending along a length of the crucible; and and aplurality of thermal shields overlying the crucible and the die, thethermal shields having configuration to provide a static temperaturegradient along the die, such that temperature is at a maximum at aboutthe midpoint of the die.
 60. The melt fixture of claim 59, wherein thethermal shields have a stepped configuration.
 61. A melt fixture,comprising a crucible; a die open to and extending along a length of thecrucible; and an adjustable gradient trim device configured to adjust atemperature gradient along the length of the die.